The transparency of the Universe to Very High Energy photons - - PowerPoint PPT Presentation

1

The transparency of the Universe to Very High Energy photons

Vulcano Workshop 2010 – Frontier Objects in Astrophysics and Particle Physics – May 2010

Outline:

• Photon propagation
• Observations of distant AGN
• Physics interpretation
• Conclusions

Barbara De Lotto - University of Udine & INFN

VHE γ-rays have opened a new window on the Universe

2010

GRBs AGN Pulsars PWN μ μ-

• quasar

quasar SNRs

• rigin of

cosmic rays

cosmology cosmology

dark matter

space time

Scientific targets

Spectral characteristics

• f blazars

Cosmic background radiation

3

Intergalactic absorption of VHE photons

Around the TeV region cross section maximized for infrared/optical photons (Extragalactic Background Light)

(Heitler 1960)

maximal for ε :

( )

θ ε β cos 1 2 1

4 2

− − = E c me

Extragalactic Background Light

• Thermal emission produced by stars

and partly absorbed/re-emitted by dust during the entire history of the Universe

• two components
• Several models try to describe the

EBL Spectral Energy Distribution: main differences depending on how the evolution in time and frequency is treated n(ε,z)

Finke & Razzaque arXiv0905.1115v2

5

Attenuation

nε (ε,z) spectral energy density of background photons (EBL)

) , ( em

• bs

) ( ) , (

z E

e E z E

τ −

× Φ ≡ Φ

τ

• ptical depth

6

(An approximation)

• Neglecting evolutionary

effects for simplicity

Coppi & Aharonian ApJ 1997

τ(E,D) ≈ D Λ(E) Λ ∝ 1 σ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ Φobs(E,D) ≈ Φem(E) × e

− D Λ(E )

For γ-rays energies above a few TeV, the distance they can propagate ≤ 100 Mpc most of the VHE Universe seems not visible to us

Λ (Mpc)

7

Consequences

• Since Λ

becomes < RHubble for E > 100 GeV:

– The observed flux should be exponentially suppressed at VHE the observed spectrum should be steeper than the emitted one. – The observed flux should be exponentially suppressed at large distances very far-away sources should become invisible as energy increases

γ – ray horizon: τ(E,z) = 1

Fazio & Stecker 1970

~ E-2

τ = 1

8

The far away VHE Universe …

38 Sources …

PKS 0447-439 z=0.20 HESS 2009 1ES 1011+496 z=0.21 MAGIC 2007 1ES 0414+009 z=0.29 HESS & Fermi 2009 S5 0716+71 z=0.31±0.08 MAGIC 2009 1ES 0502+675 z=0.34 VERITAS 2009 3C 66A z=0.44 VERITAS 2009 3C 279 z=0.54 MAGIC 2008

9

The distant quasar 3C 279

• Flat spectrum radio quasar at z = 0.54
• Very bright and strongly variable

– Brightest EGRET AGN – Gamma-ray flares in 1991 and 1996. Fast time variation (~ 6hr in 1996 flare)

• MAGIC observations

– 10 h between Jan.-April 2006 – clear detection on 23rd Feb. at 6.2σ First FSRQ in TeV γ-rays Major jump in redshift Science 320 (2008) 1752

10

Follow up observations of 3C 279

• New observations after optical outburst in Jan. 2007

new flare detected

Most distant object ever detected at VHE - two flares

(Feb. 2006 and Jan. 2007) Hard spectrum confirmed

• E. Carmona – HEAD2010

11

Implications on Extragalactic Background Light

e-

γVHE

• Power law Γ

= 4.1 ± 0.7, measured up to 0.5 TeV Spectrum sensitive to 0.2 - 2 μm

• Assume minimum reasonable index Γem

= 1.5 Upper limit close to lower limit from galaxy count Emission harder than expected

Universe more transparent

to γ-rays than expected The measurement of spectral features permits to constrain EBL models:

Γ

e+ γEBL blazar IACT e-

12

• At ~ 0.5 TeV, flux attenuated by 2
• rders of magnitude

even for the lowest EBL model:

• Possible explanations:

– from standard ones

• very hard emission mechanisms with intrinsic slope < 1.5 (Stecker 2008)
• Very low EBL

– to possible evidence for new physics …

Could it be seen?

Is there a new land just behind the horizon?

14

Oscillation to an Axion-Like Particle

( )a

B E M

a

⋅ = 1

γ

L

• available constraints in the parameters m, M:

CAST exp. & astrophysical arguments: M > 1010 GeV, m < 0.01 eV (PDG 08)

• Intergalactic magnetic field:

domain-like structure with strength ~ 0.5 nG, coherence length λ ~ 10 Mpc, random orientation in each domain RESULTS: Small mass , small coupling (within limits) naturally explain the enhancement at large E

during the propagation in the

intergalactic medium [DeAngelis,Roncadelli& Mansutti, PLB2008, PRD2008]:

m <<10−10eV (maximal mixing) 1011 GeV < M < 1013 GeV

in the presence of magnetic fields. Several interpretations:

… continue

Simet, Hooper & Serpico, PRD 08

• Conversion at the emission
• Milky Way acts as a converter to

photons

Sanchez-Conde et al., PRD 09

• Mixing inside the source and in the

intergalactic magnetic field simultaneously considered in the same framework

16

Experimental input from more sources at high z:

MAGIC S5 0716+714

• MAGIC discovery 23rd – 25th April, Atel 29th
• On 28th Swift reports high flux (0.3-10 KeV)
• MAGIC flux(>400 GeV) ≈25% Crab,

Γ = 3.45±0.54

• 3rd low-peaked VHE blazar after BL Lac & W

comae

• Host galaxy detected: z = 0.31±0.08 => 3rd

farthest VHE emitter for which the spectral index has been measured Optical light curve: KVA telescope, La Palma

ApJ 704 (2009) L129

• 3rd MAGIC discovery after optical ToO

Spectral characterstics of observed AGN: a synoptic view

17

PKS 0447-439 z=0.20 HESS 2009 1ES 1011+496 z=0.21 MAGIC 2007 1ES 0414+009 z=0.29 HESS & Fermi 2009 S5 0716+71 z=0.31±0.08 MAGIC 2009 1ES 0502+675 z=0.34 VERITAS 2009 3C 66A z=0.44 VERITAS 2009 3C 279 z=0.54 MAGIC 2008

…and more new sources are coming into the game (VERITAS, HESS, MAGIC) spectral index redshift z

Where do we stand ?

• Recent gamma observations might present substantial challenges to the

conventional models to explain the observed source spectra and/or EBL density. – MAGIC 3C279 at z=0.54; VERITAS detection above 0.1 TeV from 3C66A (z=0.44): EBL-corrected spectrum harder than 1.5 (Acciari+, ApJ09);

• TeV photons coming from 3C 66A? (Neshpor+98; Stepanyan+02). Difficult to explain with

conventional EBL and physics.

– The lower limit on the EBL at 3.6 μm was recently revised upwards by a factor ∼2, suggesting a more opaque Universe (Levenson+08). Some sources at z = 0.1 − 0.2 seem to have harder intrinsic energy spectra than previously anticipated (Krennrich+08). – Spectral indices don’t grow with increasing distance: selection bias?

• While it is still possible to explain the above points with conventional physics,

the axion/photon oscillation could naturally explain these puzzles

18

19

Other possible explanations related to new physics

• Kifune 2001: Violation of the Lorentz

invariance “a la Coleman-Glashow”:

the absorption mean free path of VHE γ – rays is altered by orders of magnitude from those conventionally estimated.

c2p2 = E2 1+ξ E Es +O E Es ⎛ ⎝ ⎜ ⎞ ⎠ ⎟

2

⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥

but

we should keep in mind that

20

Conclusions

• VHE γ

–rays have opened a new window in the Universe

• The observation of VHE γ

–rays from extragalactic sources (like blazars) has entered a new era, thanks to the new generation

• f ground based Cherenkov telescopes (and Fermi)
• To disentangle attenuation due to EBL from intrinsic properties
• f blazars, and eventually probe certain aspects of fundamental

new physics, the detection of more sources at different redshifts is essential

• We have the tool: increased sensitivity of the upgraded current

telescopes (like MAGIC) in synergy with Fermi, and possibly a future Cherenkov Telescope Array system (CTA project).

21

”A textbook example of the merging of particle physics and astronomy into the modern field of astroparticle physics”

22

BACKUP

Imaging Air Cherenkov Technique Imaging Air Cherenkov Technique

~ 10 km Particle shower

~ 1o

Cherenkov light

~ 120 m

Gamma ray Cherenkov light Image of particle shower in telescope camera

• reconstruct:

arrival direction, energy

• reject hadron background

statistically in the analysis

• Axions were postulated to solve the strong CP problem in the 70s.
• Good Dark Matter candidates (axions with masses ≈

meV-μeV could account for the total Dark Matter content).

• They are expected to oscillate into photons (and viceversa) in the presence of

magnetic fields: Photon/axion oscillations are the main vehicle used at present in axion searches (ADMX, CAST…). Some astrophysical environments fulfill the mixing requirements AGNs, IGMFs M11 : coupling constant inverse (gαγ /1011 GeV) BG : magnetic field (G) spc : size region (pc) with

15⋅ BG ⋅ spc M11 ≥1